Fix glibmm dependency check

Fix glibmm dependency check

https://sigrok.org/gitweb/?p=pulseview.git;a=commit;f=CMakeLists.txt;h=33c5ac28f73aa35c5fc4bbcf69a5ae61c0b50989 introduced the line

pkg_check_modules(GLIBMM_2_4 glibmm-2.4>2.28.0)

but https://cmake.org/cmake/help/v3.0/module/FindPkgConfig.html states

A <MODULE> parameter can have the following formats:

{MODNAME}            ... matches any version
{MODNAME}>={VERSION} ... at least version <VERSION> is required
{MODNAME}={VERSION}  ... exactly version <VERSION> is required
{MODNAME}<={VERSION} ... modules must not be newer than <VERSION>

Hence, ">" is an invalid atom and always makes the check fail. On systems without glibmm-2.68, this leads to glibmm not being found and the build aborting.